Optical torque on a two-level system near a strongly nonreciprocal medium

Abstract

We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and non-resonant part, associated respectively with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green function. Our formulation is valid for any inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. We apply this general theory to a scenario in which the atom interacts with a material characterized by strong nonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is represented by the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevant physical insight into the role of these unidirectional surface waves in the emergence of non-trivial optical torque, we derive closed-form expressions for the induced torque under the quasi-static approximation. Finally, we investigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to align due to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydberg atoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may find broad application in the context of nano-mechanical and bio-mechanical systems.

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• Inducing and controlling rotation on small objects using photonic topological materials
• Equilibrium forces on non-reciprocal materials
• Transfer of angular momentum of guided light to an atom with an electric quadrupole transition near an optical nanofiber
• Topological Materials for Near-Field Radiative Heat Transfer
• Peak, valley and intermediate regimes in the lateral van der Waals force
• Torque of guided light on an atom near an optical nanofiber
• Spontaneous Symmetry Breaking of Time-Reversal-Symmetry and Time-Crystal States in Chiral Atomic Systems
• Enhancement of Quantum Excitation Transport by Photonic Nonreciprocity
• Regimes of the lateral van der Waals force in the presence of dielectrics
• Spontaneous Rotational Symmetry Breaking in a Kramers Two-Level System
• Sign inversion in the lateral van der Waals force between an anisotropic particle and a plane with a hemispherical protuberance: an exact calculation
• Non-Markovian Transient Casimir-Polder force and population dynamics on excited and ground state atoms: weak and strong coupling regimes in generally non-reciprocal environments